THE EFFECTS OF CROP DIVERSIFICATION ON HERBIVOROUS INSECTS - A METAANALYSIS APPROACH

1. We used a meta-analysis to evaluate the hypothesis that diversified crops are subject to lower density of herbivorous insects. This method consists of integrating findings of independent studies by calculating the magnitude of treatment effects (effect size). 2. A literature review covering a period of over 10 years yielded twenty-one studies that contained sufficient information for the meta-analysis. The data were analysed according to three criteria: yearly replications of the same study were considered independent data so that research repeated over time would have greater weight; a second meta-analysis was conducted with results of only 1 year because of possible lack of independence between results of the same study; the third approach was to remove studies involving Brassica spp. from the analysis because they are redundant from the standpoint of testing a general hypothesis of crop diversification. 3. The three analyses resulted in effect sizes of 0.50, 0.35 and 0.27, respectively, and all were statistically significant. These results correspond to 69%, 63% and 60% differences between average insect densities for treatment (diversified crops) and control groups. 4. We considered the last criterion for data analysis as the most appropriate because of lowest bias from non-independence between samples. Therefore we concluded that crop diversity caused moderate reduction of herbivorous insect populations. 5. The implications and limitations of meta-analyses are discussed.